This article describes a novel phase measurement technique to increase the measurement velocity compared to the previous arc-tangent method in the heterodyne laser interferometer. The proposed method can reduce the calculation load because the pulse width modulation signal has a linear relation between the phase difference, while the nonlinear function such as arc tangent is required to demodulate the sinusoidal interferent signal. The brief analysis and measurement scheme of the system, and the experimental result using a Zeeman-stabilized He–Ne laser are presented. They demonstrate that the proposed phase measurement technique is proven to be three times faster and more robust than previous arc-tangent method.

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The homodyne interferometer has the advantages of simple optical configuration and low cost. However, it requires a caution in using an electronic filter such as high pass filter (HPF) to get rid of low frequency electronic noises and the dc offset associated with the optical intensity in the photodiode electronic circuitry. When the vibration amplitude is smaller than at least 12 of the wavelength of He-Ne laser, a problem of incorrect velocity or distorted velocity measurement can be caused since a dc value of the interference signal is eliminated by using the HPF. To solve this problem of using the HPF in the homodyne interferometer, a synthetic interferometer using a mechanical modulation method is proposed in this work by exciting a reference mirror with the displacement larger than 12 of the wavelength. In this work, the analytical work is presented to show how the synthetic interferometer solves the problem of incorrect velocity measurement by using the Fourier-Bessel function description of the interference signals. Simulation and experimental works are also presented to validate the synthetic heterodyne interferometer proposed in this work.

The signal processing algorithm for a position sensitive detector (PSD) using an amplitude modulation/demodulation algorithm is addressed in this article. Dark currents and the external noises such as daylight and fluorescent lights are eliminated by using the amplitude modulation/demodulation and a low pass filter. The proposed amplitude modulation/demodulation method for the PSD sensor is compared with a pulse amplitude modulation method (PAM). The proposed amplitude modulation method is proved to be more accurate and robust than the PAM method by using analysis and experiments. Multiple measurements technique using one PSD sensor by the amplitude modulation/demodulation is also addressed. The power variation of light source is compensated by the normalization process using a divider.